1. Field of the Invention
The subject matter disclosed relates in general to the measurement of size and mass concentration of particles. More specifically, it relates to apparatuses and methods of making concentration and mean agglomerate particle size measurements using scattered light.
2. Description of the Related Art
The ability to measure and to quantify the behavior and characteristics of particles (e.g., measurement of individual size and local number and/or mass concentrations) is of utmost importance in a large number of applications of interest such as, for example, industrial boilers and furnaces, different processes in petroleum refineries, food preparation, and pollution of the environment caused by industrial and vehicular emissions. The need to perform particle-related measurement is further evidenced by the large number, and ever more stringent, governmental regulations dealing with the subject. One of the challenges in making these measurements relates to the fact that particle sizes (“small” or “large”) and concentrations (dilute or dense flows) vary widely depending on the specific application. For example, during the combustion process inside a boiler or furnace one may be concerned with particle sizes varying from 1 mm down to 1 μm with concentrations ranging from 1 mg/m3 to 50 g/m3; however, when dealing with atmospheric pollution, the sizes of interest may be much smaller (e.g., ranging from 10 μm to 10 nm) and concentrations can be on the order of 1-1000 μg/m3. Yet, in some other applications, the interest may be in even broader size distributions, as for example, particle sizes from 1 mm down to 1 nm. The technological challenge is how to make reliable measurements of this large size range under such a wide range of concentrations. There are many different instruments available to make measurement of particle size; however, laser-based techniques are the primary alternative for real-time measurement in dilute flows of very small particles and have provided the most reliable results. They typically include single-particle counters as well as ensemble instruments that measure multiple particle scattering. Examples include opacity, angular light diffraction, dynamic particle arrival fluctuations, and large-angle, phase-detected nephelometry. In general, the smaller the particle size and the more dilute the flow, the more difficult the measurement will be due to decreased scattering signals.
In laser-based ensemble instruments, a laser light source is provided and focused or collimated to form a probe volume and the light scattered, absorbed, and/or transmitted by the particles flowing through this probe volume is measured, using one or several light detectors. Light detectors are devices that produce a voltage or current signal that is related to the amount of light incident on the detector's face. Given the intermittent nature of the particles flowing in any system, detector signals naturally vary as a function of the particle number, size, and residence time in the probe volume.
As an example, classical nephelometry, or right angle scattering, measures the time-averaged scattering signal from a large number of particles (typically greater than 100 particles in a probe volume). For general types of particles in the size range of 0.2 to 1 micron, the scattering signal is roughly proportional to the particle concentration alone, while outside this range, the scattering signal becomes strongly dependent on both the particle size and concentration. In another example, opacity measurements determine the average amount of light that is absorbed and scattered out of a probe volume. For particles larger than 1-2 microns, this results in a simple relationship between the signal attenuation and the ratio of concentration to mean particle size. However, in both of these examples, a single scattering measurement does not provide independent information on both the particle size and concentration.
Another challenge in performing the above-summarized measurements and others is the fact that optical scattering properties are normally dependent on the type of particle being measured. For example, combustion soot has unique optical scattering properties. Many optical measurement techniques have been developed over the years that measure light scattering at one or more angles, or use absorption methods to determine the mass concentration of particles, as for example, black carbon soot. The decreasing concentration levels occurring in practical combustion engines limit absorption methods. Light scattering methods have invariably required the use of empirical calibrations to relate the scattering measurement to the desired mass concentration or particle size. Solution of the light-scattering equations in conventional two-angle scattering measurements provided by De Luliis et al. (De Luliis, S., Cignoli, F., Benecchi, Zizak, G. “Determination of Soot Parameters by a Two-angle Scattering-extinction Technique in an Ethylene Diffusion Flame,” Applied Optics, 37(33): 7865-7874 (1998), the contents of which are herein incorporated by reference in their entirety) are based on the explicit dependence of particle light scattering properties on geometrical properties of the particle structure.
Therefore, it is desirable to have an optical instrument to quantitatively measure the mass concentration and mean particle size using light scattering measurements to give fundamental size and concentration measurements in a way that is based on fundamental theory without empirical calibration so as to provide a formulation that gives a direct and straightforward interpretation of the size and concentration in terms of the measured scattering signals, the instrument properties, and the particle optical properties. As used hereinafter, the expression “particles” refers to any agglomerates made up of similarly sized primary particles, such as, for example, soot.